I. Field of the Invention
The present invention relates to communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for selecting a preferred communication system in a subscriber station capable of operation in a plurality of geographical regions.
II. Description of the Related Art
As mobile communication systems become more prevalent in society the demands for greater and more sophisticated service have grown. To meet the capacity needs of mobile communication systems, techniques of multiple access to a limited communication resource have been developed. The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems.
The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, assigned to the assignee of the present invention, and is incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention and is incorporated by reference herein.
When the user of a subscriber station travels from one geographic area to another, the subscriber station must select a communications system upon which to conduct services. There are two means by which a user may operate his subscriber station in differing geographic regions. By the first method, the user subscribes to communications services in a variety of regions. Thus, the subscriber station needs only to seek out a communications system to which the user subscribes and may provide services using any of those service providers.
Alternatively, the user may communicate by means of roaming service. Mobile communications providers negotiate contracts among themselves to provide services known as xe2x80x9croamingxe2x80x9d to their customers. A xe2x80x9croamerxe2x80x9d is a subscriber station which requires service in a system which is operated by a mobile communications service provider other than the ones to which the user subscribes. Currently, when a subscriber station is roaming, a signal indicative of the roaming condition is provided to the user. A roaming determination is made as a result of a comparison of the system identification (SID) of the subscribed system or systems with the SID of the system providing service which is broadcast by that system. This alerts the user of the subscriber station that the service being provided is accruing roaming charges.
Because the subscriber station is generally without knowledge of the user""s geographic location, it must select the system (from the possible communications systems of an area) which provides the optimum service to the user in terms of cost and quality of service. As the number of regions in which the user wishes to be able to operate increases, so does the number of different communications systems that the subscriber station must attempt to acquire. The present invention provides a method and apparatus for selecting the communication system best suited to the user""s needs.
The present invention is described in a multi-mode subscriber station, such as is described in detail in U.S. Pat. No. 5,754,542 entitled xe2x80x9cMETHOD AND APPARATUS FOR SYSTEM DETERMINATION IN A MULTI-MODE SUBSCRIBER STATIONxe2x80x9d, which is assigned to the assignee of the present invention and is incorporated herein by reference. The exemplary embodiment is illustrated in the context of a subscriber station capable of transmitting and receiving CDMA signals and capable of transmitting and receiving analog signals, such as AMPS and NAMPS. The present invention is equally applicable to any digital communication system including TDMA, FDMA and GSM. In addition, the present invention is equally applicable to subscriber stations capable of operation in only one mode (e.g. analog or digital).
In the present invention, the subscriber station maintains a list of systems, some of which are xe2x80x98preferredxe2x80x99 systems (systems the subscriber station is allowed to use), and some of which are xe2x80x98negativexe2x80x99 systems (systems the subscriber station is not allowed to use). Associated with each system in the list is a system ID (SID), as well as corresponding acquisition parameters (band, frequency, mode, etc.). This list is referred to herein as the universal system table.
The universal system table is maintained in such a manner that the subscriber station can readily determine which systems (preferred or negative) cover common geographical regions. The references to common geographic regions refers to areas of common radio coverage. Moreover, the systems that cover a common geographical region are prioritized (i.e., ranked from most desirable to least desirable). The subscriber station""s job is to attempt to acquire service on the most desirable system in the subscriber station""s current geographical area. There is no point in trying to acquire service on a system outside of the subcriber station""s current geographic region, since systems typically provide service only within a limited geographic region.
The problem is that the subscriber station does not necessarily know where it is when it powers on. Due to roaming, it could be in an entirely different region than it was previously. Therefore, it may not be obvious how to acquire any system, let alone the most desirable system. In the exemplary embodiment of the present invention, the subscriber station maintains a list of the most recently used (MRU) systems. As a first measure, attempting to acquire one of these systems makes a lot of sense, because there is a good chance that the subscriber station is currently somewhere that it has been in the not too distant past.
If the subscriber station cannot acquire an MRU system, then it can try to acquire any preferred system in the universal system table, using the appropriate acquisition parameters. In the exemplary embodiment, the subscriber station first attempts acquisition on the xe2x80x98easiestxe2x80x99 to acquire systems, (e.g., AMPS systems assuming there are any AMPS systems in the universal system table). In the exemplary embodiment, if the subscriber station is unable t5o acquire one of these systems, then the subscriber station attempts to acquire system which is xe2x80x98most representativexe2x80x99 of systems in the universal system table. In other words, the universal system table will probably contain numerous systems which differ only with respect to their SIDs and their geographical regions, but having identical acquisition parameters (e.g., channel number or frequency block designator).
Once the subscriber station acquires a system, the subscriber station can pick up the acquired system""s SID from an overhead message. The subscriber station uses the received SID to determine the geographic region in which it is located. In the exemplary embodiment, the subscriber station can obtain this geographic information from the universal system table whether the acquired system is preferred or negative.
If the SID belongs to a preferred system in the universal system table that is a most desirable system within its geographical region, the subscriber station provides service using that system. If the SID of the acquired system belongs to a system in the universal system table that is not a most desirable system within its geographical area, the subscriber station uses the received SID to determine the geographic region in which it is located. The subscriber station attempts to acquire a most desirable system in that geographical area, by sequentially making acquisition attempts on systems in the geographic region from most desirable to least desirable for the region.
If the SID belongs to a system not in the universal system table, the subscriber station attempts acquisition on other systems. The subscriber station may return to this system if the subscriber station fails to acquire a preferred system.